Early Miocene spore and pollen record of the Gaiman Formation (Northeastern Patagonia, Argentina): correlations and paleoenvironmental implications

PALAZZESI, LUIS; BARREDA, VIVIANA; SCASSO, ROBERTO A.

Bariloche

Congreso; 4º Congreso Latinoamericano de Sedimentología y 11º Reunión Argentina de Sedimentología; 2006

Asociación Argentina de Sedimentología

Fossil pollen preserved in shallow marine beds of north-eastern Patagonia (Gaiman Formation, Valdés Basin) was used to date and correlate sedimentary strata cropping out in the area. The temporal assignment of these deposits has been controversial; were dated as Eocene or Late Oligocene-Early Miocene, depending on the fossil record considered and the author involved. Outcrops near Puerto Pirámide (Eje Tentativo section) and Trelew localities (Autódromo section) were sampled and analysed for palynological purposes. The pollen and spores content recovered from these deposits was compared to that of the palynological zones up to now recognised for Patagonia. The assemblages contained dinoflagellate cysts, pollen and spores as well as foraminiferal linings and plant tissue remains. Pollen and spores recovered were frequently corroded and selective preservation was observed. The occurrence of some stratigraphically and ecologically important fossil species provided a better understanding of the age and paleoenvironmental conditions during their accumulation, though. Major differences is taxonomic composition were recognised between the sediments analysed from E.T.S (Eje Tentativo section) and A.S. (Autódromo section). The age determination of the E.T.S. is based on the occurrences of Glencopollis ornatus Pocknall and Mildenhall and some specimens of Tubulifloridites antipodica Cookson, species not recorded before Miocene times. Moreover, Microcachrydites antarcticus Cookson, Arecipites subverrucatus (Pocknall) Mildenhall and Pocknall and Haloragacidites trioratus Couper were not recorded in Patagonia from late Early Miocene onwards. These species characterize the G palynological zone of Early Miocene age in deposits of the San Jorge Gulf Basin and Austral Basin. The age of the A. S. is defined by the presence of Baumannipollis chubutensis Barreda, the most prominent species recovered which defines the T-B palynological zone of the upper beds of the Chenque Formation. Even when all these data are preliminary, and probably need further studies, these two major palynological assemblages with significant compositional differences revealed that E.T.S. and A.S. were accumulated during the Miocene. In addition, absence of the T-B palynological zone in E.T.S., at the top of the local section of the Gaiman Fm., points to deep erosion prior to sedimentation of the overlying Pto. Madryn Fm. (early Late Miocene). The ratio between the continental and marine palynomorphs recovered from both sections also suggests that they were deposited in a shallow marine  environment, near the coast line. The A.S. assemblage, with an abundant content of swamp taxa, would have originated in a coastal lagoon environment. The E.T.S. showed higher frequencies of marine elements, and so a further distance from the coast line is suggested.